The chemistry behind the application of ionizing radiation in water-pollution abatement

This chapter discusses the chemistry behind the application of ionizing radiation in water-pollution abatement. Ionizing radiation possesses several attractive features as a means to induce the destruction, that is, in the limit, the mineralization of aqueous organic pollutants. The chapter emphasizes on the chemical-mechanistic aspects of the radiolytic decomposition of certain classes of environmentally noxious chemicals in the aquosphere. The chemistry at the basis of the technology consists in the reactions of the free radicals OH, H, and hydrated electron eaq-, created in the proportion of about 5/1/5 upon the radiolysis of the polluted water, with the pollutant solutes. In the abatement of aromatic pollutants, the main initial transient species is the substrate-OH-radical adduct, hydroxycyclohexadienyl. Depending on the nature of the substituents in the ring, the addition of dioxygen to these radicals may be reversible. Treatment of aromatics with ionizing radiation in order to degrade them is not necessarily always entirely beneficial. Thus, with an effluent that produces NO2, traces of nitro derivatives can be formed by free-radical recombination. These compounds are usually very resistant to biodegradation. Hydroxylated aromatics are among the intermediates in the deep degradation of aromatic hydrocarbons. In addition, they form a class of pollutants of their own. Phenoxyl radicals are important intermediates in their oxidative oxidation.